1. Field of the Present Invention
The present invention relates to a control device for self-exciting current source power converter.
2. Description of the Related Art
FIG. 1 is the drawing showing the constitution of main current of self-exciting current source power converter of the prior art.
In FIG. 1 the reference 101 indicates an alternating current load and references 102-104 indicate capacitors. References 105-108 indicate unit converters (hereinafter mentioned simply as converters), which are connected to AC load by unifying the respective AC terminals commonly to perform a parallel operation.
Also the capacitors 102-104 are used for absorbing the switching surge of the converters 105-108. The devices 109-132 are the self-turn-off devices forming the converters 105-108. Hereinafter, the cases, in which gate turn-off thyristors (hereinafter called GTO) are used as self-turn-off devices, are explained.
References 133-140 indicate the DC reactors which make DC smooth. References 141-144 indicate the DC electric sources. Each of the electric sources 141-144 is provided with a function to regulate the DC current, and the respective currents are equally regulated.
FIG. 2 shows the block diagram of the regulating circuit of the prior art for regulating the power converter of FIG. 1. Referring to FIG. 2, reference 149 indicates a circuit generating the current command value for the converters 105-108, reference 150 indicates a triangular wave generator, reference 152 indicates a comparator, and reference 153 indicates a logical circuit generating AC output commands for the converters 105-108.
FIG. 3 shows wave-diagrams, when the power converters of FIG. 1 is regulated by the regulating circuit of prior art shown in FIG. 2. Hereinafter the power converter of prior art is explained by referring to FIG. 1 and FIG. 2. The circuit generating electric current command value 149 generates a amplitude command value (1) and a phase angle command value. The amplitude command value (1) is given to the comparator 152.
The phase angle command values are given to both the phase angle detecting circuit 150 and the triangular wave generator 151. The signals (2)-(5) are the output signals forming triangular waves from the triangular wave generator 151, which period corresponds to 60.degree. of the phase angle command values. Also, each of the triangular waves (3)-(5) delays by 15.degree. each other compared to the triangular wave (2). The comparator 152 compares the amplitude command value (1) with the triangular waves (2)-(5), and the range, in which the amplitude command value is larger than the triangular waves, is regarded as output command. The output command is given to the logical circuit 153, and its phase identification determined by the output of the phase detecting circuit 150 and the AC output command for the converter is generated.
In other words, (6) is the U-phase output command of the converter 105 and (7) is the X-phase output command for the converter 105, (8) is the U-phase output command for the converter 106, (9) is the X-phase output command for the converter 106, (10) is the U-phase output command for the converter 107, (11) is the X-phase output command for the converter 107, (12) is the U-phase output command of the converter 108, and (13) is the X-phase output command for the converter 104.
By ON-OFF controlling the GTO of the converters 105-108 according to the output commands mentioned in the above, the current of the wave-form shown in (14) is obtained as U-phase output current. The same regulation as in the above is performed also for the V-phase and W-phase by delaying the respective phases by 120.degree. each.
As described in the above, when the operation is performed by delaying the phase of the current of the GTO by 15.degree. each by connecting the AC terminals of the converters 105-108 in parallel, the wave form of the composite wave of the waves from the respective converters becomes trapezoid wave containing the high frequency harmonic waves of the lower orders such as 5th and 7th order.
Also, in case of the self-exciting current source power converters in FIG. 1, as the electricity may be commutated in an arbitrary phase, the active electric power and the reactive electric power may be controlled independently, however, in case of the self-turn-off devices such as GTO, a minimum ON-time (or minimum OFF-time, for example 200 .mu.s) has to be held as shown at (a), (b) and (c) of FIG. 6. Therefore, a pulse which is shorter than the minimum holding time maynot be outputted, and the power maynot be outputted in the inoperable range (d), i.e., in the range of 0 PU (Zero-par-unit).